The 3rd dB: Why a Lossy Attenuation Network Pad Works Well With RF ADCs
Abstract
One common question received from customers using an RF ADC, particularly when developing a passive analog frontend design, for example, using a balun or transformer to couple to the ADCs analog input pins, is: why is there a resistive 3 to 6db attenuation network between the balun and ADC? This creates a lot of signal loss! What is the purpose of this three-resistor network? Six actually, since the input is differential.
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1 Introduction
Start with a simple block diagram setup as shown in Figure 1-1 with a balun, attenuation pad and ADC. This line up is typical of what you can find on a standard high-speed or RF ADC evaluation board. The attenuation pad is the component in controversy; on why does a designer implement a frontend network in this manner? Why not simply connect the balun outputs directly to the ADC's input pins? The ADC data sheet shows that the ADC's input impedance is 100Ω differential in most cases, that is, for internal input buffered style high-speed ADCs.
Figure 1-1 Generic ADC Signal Chain Block Diagram with Attenuation Pad